In general, the drawbacks resulting from the frictional resistance to high-speed of the sliding bearings led to the development of roller bearings. Roller bearings are composed of a pair of steel rings (rails) separated by one or more rows of balls or rollers (rolling bodies), which eliminate the sliding friction. The balls are kept equidistant to each other through a cage to distribute forces and maintain concentricity of the rings, the outer ring is fixed to the wheel hub and the inner ring is attached directly to the axis, or shaft. The bearings can interact with two types of loads: radial and axial. The radial force is that extending or moving from a central point outwardly and the axial force is that extending or dissipating through the central axis.
The roller bearing housing in the hub cavity requires machining processes of high dimensional accuracy so that the bearing does not move radially inside the cavity. For this reason, the dimensional variation is usually determined by the bearing supplier. However, the conventional processes for obtaining the bearing hub do not ensure the dimensional repeatability required by the system without further accuracy finishing, typically grinding, resulting in difficulty of assembly or excessive clearance of the bearing in the hub. In this case the outer track of the bearing rotates within the cavity with a high possibility of noises and wear, reducing the lifetime of the product.
Typically, in a construction of single-phase or three-phase induction motors with squirrel cage rotor, the air gap value should be sized according to the machine design and a series of combined data. A text, published in 1936, states that “ . . . it is necessary (to lower the magnetizing current) using a small air gap (but not too small) . . . . The [clearance] must be chosen such that the excitation current and reactance of the machine are in accordance with the desired performance. Reduced gaps can increase motor noise and losses in the tooth face . . . ”
The existing variations in the air gap will depend on the slit dimensions, stator and configuration of the winding. In any case, an air gap variation up to 20% does not result in a significant change in the performance of the locked rotor, a fundamental aspect of a motor's performance.
In commercial ventilation applications utilizing fractional horsepower motors, i.e., with powers lower than 1 cv, the radial loads are usually proportionally small when compared to the mechanic resistance of the viscoelastic bearing. In these conditions, the bearing barely touches the seat of the wheel hub in normal application, maintained only by the elastic action of the viscoelastic bushing, which causes the assembly to operate advantageously in relation to the noise. Usually, it is desired that the noise is low in this type of application when compared to others conventional applications of rotating electrical machines. In this application, we typically find air gap values between 0.2 to 0.35 mm in products normally commercialized.
In particular, applications in rotating electrical machines require accuracy and alignment of the assembly. The wheel hubs of roller bearings are usually machined because standards require controlled values for durability, noise, and vibration.